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26 .\" $FreeBSD: src/share/man/man4/da.4,v 1.22.2.7 2001/10/01 13:07:23 dd Exp $
33 .Nd SCSI Direct Access device driver
36 .Cd device da1 at scbus0 target 4 unit 0
40 driver provides support for all
42 devices of the direct access class that are attached to the system
46 The direct access class includes disk, magneto-optical,
47 and solid-state devices.
52 adapter must also be separately configured into the system
55 direct access device can be configured.
59 driver allows the disk to have two levels of partitioning.
62 is used to separate the
64 areas of the disk from areas used by other operating systems.
65 The second layer is the native
69 which is used to subdivide the
71 slices into areas for individual filesystems and swap spaces.
72 For more information, see
78 If an uninitialized disk is opened, the slice table will be
79 initialized with a fictitious
81 slice spanning the entire disk. Similarly, if an uninitialized
84 slice is opened, its disklabel will be initialized with parameters returned
85 by the drive and a single
87 partition encompassing the entire slice.
89 Many direct access devices are equipped with read and/or write caches.
90 Parameters affecting the device's cache are stored in mode page 8,
91 the caching control page. Mode pages can be examined and modified
96 The read cache is used to store data from device-initiated read ahead
97 operations as well as frequently used data. The read cache is transparent
98 to the user and can be enabled without any adverse effect. Most devices
99 with a read cache come from the factory with it enabled. The read cache
100 can be disabled by setting the
102 (Read Cache Disable) bit in the caching control mode page.
104 The write cache can greatly decrease the latency of write operations
105 and allows the device to reorganize writes to increase efficiency and
106 performance. This performance gain comes at a price. Should the device
107 lose power while its cache contains uncommitted write operations, these
108 writes will be lost. The effect of a loss of write transactions on
109 a file system is non-deterministic and can cause corruption. Most
110 devices age write transactions to limit vulnerability to a few transactions
111 recently reported as complete, but it is none-the-less recommended that
112 systems with write cache enabled devices reside on an Uninterruptible
113 Power Supply (UPS). The
115 device driver ensures that the cache and media are synchronized upon
116 final close of the device or an unexpected shutdown (panic) event. This
117 ensures that it is safe to disconnect power once the operating system
118 has reported that it has halted. The write cache can be enabled by
121 (Write Cache Enable) bit in the caching control mode page.
125 device driver will take full advantage of the SCSI feature known as tagged
126 queueing. Tagged queueing allows the device to process multiple transactions
127 concurrently, often re-ordering them to reduce the number and length of
128 seeks. To ensure that transactions to distant portions of the media,
129 which may be deferred indefinitely by servicing requests nearer the current
130 head position, are completed in a timely fashion, an ordered tagged
131 transaction is sent every 15 seconds during continuous device operation.
132 .Sh BAD BLOCK RECOVERY
133 Direct Access devices have the capability of mapping out portions of
134 defective media. Media recovery parameters are located in mode page 1,
135 the Read-Write Error Recovery mode page. The most important media
136 remapping features are 'Auto Write Reallocation' and 'Auto Read
137 Reallocation' which can be enabled via the AWRE and ARRE bits,
138 respectively, of the Read-Write Error Recovery page.
139 Many devices do not ship from the factory with these feature enabled.
140 Mode pages can be examined and modifie
144 .Sh KERNEL CONFIGURATION
145 It is only necessary to explicitly configure one
147 device; data structures are dynamically allocated as disks are found
156 disks as well as to other disks. They are defined in the header file
157 .Aq Pa sys/disklabel.h .
159 .Bl -tag -width DIOCSDINFO
161 Usually used to set up a bad-block mapping system on the disk.
163 drives incorporate their own bad-block mapping so this command is not
166 Read, from the kernel, the in-core copy of the disklabel for the
168 This may be a fictitious disklabel if the drive has never
169 been initialized, in which case it will contain information read
174 Give the driver a new disklabel to use.
178 disklabel to the disk.
180 Enable or disable the driver's software
181 write protect of the disklabel on the disk.
183 Give the driver a new disklabel to use.
186 write the new disklabel to the disk.
189 If a device becomes invalidated (media is removed, device becomes unresponsive)
190 the disklabel and information held within the kernel about the device will
191 be invalidated. To avoid corruption of a newly inserted piece of media or
192 a replacement device, all accesses to the device will be discarded until
193 the last file descriptor referencing the old device is closed. During this
194 period, all new open attempts will be rejected.
196 .Bl -tag -width /dev/rsdXXXXX -compact
197 .It Pa /dev/rda Ns Ar u
202 accessed as an unpartitioned device
204 .It Pa /dev/da Ar u Pa s Ar n
212 accessed as an unpartitioned device
214 .It Pa /dev/rda Ar u Pa s Ar n
222 accessed as an unpartitioned device
223 .It Pa /dev/da Ns Ar u Ns Ar p
232 .It Pa /dev/rda Ns Ar u Ns Ar p
284 driver was written for the
288 .An Justin T. Gibbs .
289 Many ideas were gleaned from the
291 device driver written and ported from
295 .An Julian Elischer .
296 Support for slices was written by